In general, active baluns are unidirectional converters between differential and single-ended signals, and passive baluns are bi-directional converters between differential and single-ended signals. Active baluns are typically used for applications requiring large bandwidth. The bandwidth's upper-end frequency of an active balun depends on the technology's speed, while the lower-end frequency, although limited by DC-decoupling capacitors in the ports, can potentially extend to DC. Such unbounded lower-end frequency enables active baluns to be realized in limited chip-areas, which enables the active baluns to be suitable for Built-in Self Test (BIST) applications, as well as general differential circuits requiring compact sized broadband baluns.
Of interest for radio frequency, millimeter wave and other high frequency applications are the bandwidth capabilities, size, and cost of a particular balun configuration.
In a given CMOS or Bipolar process, the distributed amplifier configuration produces the largest bandwidth. In a distributed amplifier, the gain-bandwidth product is increased by paralleling several FETs without paralleling their input or output capacitances, thus achieving operation over extremely wide bandwidths. However, the distributed amplifier configuration occupies a fairly large area of at least 1 mm×2 mm, which increases costs. A suitable approach for limited chip-areas is the differential amplifier type configuration, which is most commonly used due to its large common-mode rejection. In a differential amplifier configuration, a common input is provided to a differential amplifier, which in turn supplies two outputs. The harmonic components can be suppressed by the amplifier due to its common mode rejection characteristics. However at high frequencies, the two outputs become unbalanced because they travel through different number of stages from the common input. A series LC-network can be used to compensate for the mismatched phase and enable gain and phase adjustment, but this sacrifices the bandwidth's lower-end frequency.
Matching the number of stages of a signal path can be achieved by using a source-drain output configuration. This still has some critical inherent problems, such as low dynamic range and the requirement of a second gain stage to simultaneously fulfill the required gain and 50 Ωmatching at each port of the differential output. The more critical problem is in the relative phase of the differential output at high frequencies, because the drain's output has an additional pole and an additional negative-zero compared to that of the source's output.
One method for achieving balanced phase at high frequencies is to use common-source and common-gate pair type. In such an active balun device, an input signal is provided to sources and gates of common gate and source FETs, and an output signal is derived at drains of the FFTs, provided a phase difference of 180° at the two outputs. However, process and parasitic components can greatly affect the bandwidth. Using this configuration, the relative phase can maintain 180° at high frequencies beyond the first few poles. However, a new challenge exists for matching the input, due to its high parasitic capacitance from driving two transistors (one for common-source path and the other one for common-gate path) that in most designs are accompanied by a third transistor from a current-source for biasing the common-gate's stage.
Accordingly, active baluns continue to require additional research and improvement for high frequency and broadband applications.